It is known that most internal combustion engines, and particularly the diesel engines, are nowadays provided with a fuel injection system. A conventional fuel injection system comprises a fuel rail and a plurality of electrically controlled fuel injectors, which are hydraulically connected with the fuel rail through respective feeding conduits. Each fuel injector generally comprises a fuel inlet, a fuel outlet and a movable needle which repeatedly opens and closes the fuel outlet, thereby injecting the fuel into the engine through a plurality of separated injection pulses.
The movable needle is actuated with the aid of a dedicated actuator, typically a solenoid actuator or a piezoelectric actuator, which is driven by an electric circuit controlled by an engine control unit (ECU). The ECU operates each injection pulse by generating an electric opening command, causing the needle to open the fuel injector, and a subsequent electric closing command, causing the needle to close the fuel injector.
The timing of the opening and closing electric commands is still controlled by the ECU, which determines two key parameters for each injection pulse, namely an electric dwell time and an electric energizing time. The electric dwell time is the time between the instant in which the electric opening command of an injection pulse is generated, and the instant in which the electric closing command of the previous injection pulse was generated.
The electric energizing time is the time between the instant in which the electric opening command of an injection pulse is generated, and the instant in which the electric closing command of the same injection pulse will be generated. The electric energizing time is generally determined by ECU as a function of the quantity of fuel to be injected in the course of the injection pulse, taking into account the value of the pressure inside the fuel rail.
However, the quantity of fuel injected in the course of a single injection pulse is not really related to the fuel rail pressure value, but it is rather related to the pressure in the fuel injector inlet at the instant in which the movable needle actually opens the fuel injector outlet, which does not always corresponds to the fuel rail pressure value. Indeed, the pressure in the fuel injector inlet is influenced by a pressure wave, which is generated by the previous fuel injection pulse performed by the same fuel injector, and which propagates along the feeding conduit connecting the fuel injector to the fuel rail, thereby producing a pressure fluctuation in the neighborhood of the fuel rail pressure value.
Starting from the end of the previous injection pulse, this pressure fluctuation progressively dampens, and stabilizes at the fuel rail pressure value after a damping period depending on the rail pressure value itself, and further depending on the fuel quantity which has been injected by the fuel injector in the course of the previous injection pulse. Therefore, this pressure fluctuation can generally be disregarded when the time between two subsequent injection pulses is sufficiently long, such as for example when the fuel injector is provided for performing one single injection pulse per engine cycle; but it must be strictly taken into account when the time between two subsequent injection pulses is very short, such as for example when the fuel injector is provided for performing a plurality of injection pulse per engine cycle.
To disregard the pressure fluctuation in this latter case could result in a deviation of the injected fuel quantity with respect to the expected one, leading to a worse fuel combustion and to polluting emissions and noise increases. In order to avoid such drawback, many strategies have been recently considered, which provide to take into account the pressure fluctuation effect. One of these strategies provides for applying a proper correcting factor to the pressure value measured in the fuel rail.
This correcting factor is determined by the ECU using an empirically determined map, which correlates the correcting factor to the electric dwell time preceding the injection pulse, to the fuel quantity injected in the course of the previous injection pulse, and to a plurality of other important engine operating parameters, such as engine speed and engine load.
Another strategy provides for modeling the pressure fluctuation according to a mathematical equation, which returns the value of the pressure in the fuel injector inlet as a function of the fuel rail pressure value and of the time elapsed from the end of the previous injection pulse. According to this strategy, the pressure value in the fuel injector inlet is therefore estimated by applying the electric dwell time and the fuel rail pressure value to the above mentioned mathematical equation.
A drawback of both these strategies is that, due to the mechanical characteristics of the fuel injectors, specially of the solenoid injectors, the instant in which the ECU generates the electric opening command does not coincide with the instant in which the movable needle actually opens the fuel injector, as well as the instant in which the ECU generates the electric closing command does not coincide with the instant in which the movable needle actually closes the fuel injector. As a consequence, the use of the electric dwell time as entry parameter in the above mentioned map or mathematical equation can still cause a significant error in the estimation of the pressure value in the fuel injector inlet, resulting in a correspondent error in the injected fuel quantity.
In view of the above, it is at least one object to provide a physical parameter capable to better represent the time period between two subsequent injection pulses performed by a fuel injector. In addition, other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.